The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Vehicle braking systems have gone through a long way of evolution from the very primitive leather frictional brake to drum brake, disc brake and coming to brakes with vacuum boost, electric power assist, as well as antilock braking system (ABS), including a brake master cylinder with central valves for good ABS performance yet not without high costs. Recent development of vehicle brake systems results in various brake-by-wire (BBW) systems, which include electro-hydraulic brake (EHB) system and electro-mechanical brake (EMB) system.
With respect to power boost of vehicle brake systems, most of the systems adopt vacuum boost and some few systems adopt electric power assist. While the electric power assist can provide a more precise control of vehicle braking over the vacuum boost counterpart, it suffers a short-coming that the manual part of the braking operation cannot be disconnected from the driver. This functional limitation prevents the braking system operation from reaching its optimal performance when regenerative braking is desired in the vehicle.
The brake-by-wire systems have various performance advantages over traditional brake systems not only in improvement of braking efficiency, braking stability and consistency, this type of system is also a necessary element for modern dynamic chassis control and new-energy vehicle braking system, especially the regenerative braking. Furthermore, the brake-by-wire system provides two critical functionalities that traditional power-assisted brake is completely lacking: (1) the BBW system can provide active braking to perform full or partial vehicle braking without depression of the brake pedal by the driver, and (2) on the other hand, with the BBW system, driver's depression of the brake pedal does not necessarily activate vehicle friction brake. These two functionalities make the BBW system superior over traditional power-assisted brake system in advanced vehicle chassis dynamic features such as acceleration slip control (ASR), electronic stability enhancement control (ESC), adaptive cruise control (ACC), and many others. However, the BBW systems arrive at these performance advantages at the expenses of system complexity and costs.
While the BBW systems offers performance superiority over the traditional power assisted brake systems, so long as the manual braking is disconnected with the by-wire braking, system fail-safe performance must become of the highest concern. The BBW systems rely on sophisticated electronic sensing and signal processing to prevent disaster from happening, including implementation of full-time monitoring and continuous self diagnosis of the system to assure a healthy system performance and avoid dangers to vehicle personnel upon system fault or failure. However, complexity of system configuration and high costs are often associated with such designs.
While there are a few different forms of BBW systems available in various platforms of automobiles, each of them has its unique performance or functional limitations. For example, in the various configurations of the EHB type BBW system, a high-pressure accumulator is needed. During the braking operation hydraulic pressure may be channeled to a master cylinder to push a piston to create pressure to vehicle wheel cylinders; or, alternatively, the hydraulic pressure may be channeled directly to the wheel cylinders. The high-pressure accumulator needs to be replenished to maintain the required hydraulic pressure even when the vehicle braking is not in action, and it may even incur risk of leakage during an accident imposing safety threats to personnel. In addition, in order to provide a good pedal feel for the driver, complicated mechanism of brake pedal simulator is often required in EHB.
In contrast with EHB, the EMB applies pressure to brake pads directly via mechanical amplification of motor torque without hydraulic circuit mechanism; thus eliminating the need of the high-pressure accumulator and hydraulic lines. However, such systems require complicated mechanical energy conversion structure to provide the required braking force. Moreover, reliability of the EMB fail-safe capability is still a challenge to many automotive manufacturers. It should also be noted that EMB system is not compatible with traditional braking mechanism. Once EMB is decided to be the vehicle brake system, all components of the vehicle brake system need to be re-designed anew, resulting in a much higher cost.
Therefore, it is the objective of the present invention to provide an electro-hydraulic brake system that possesses the two functionalities of vehicle braking that cannot be materialized in any of the state-of-the-art braking system: an electric power assist for braking operation keeping the driver in the loop and a brake-by-wire braking operation independent of driver's brake pedal command. The present invention thus demonstrates the following features:
Advantageously, the present invention of the EHB system is capable of performing both functionalities of electric power assist braking and by-wire braking operation. This EHB system possesses advantages of a non-BBW system in high degree of reliability and natural brake pedal feel, as well as advantage of a by-wire braking system in performing active braking operation;
Advantageously, the present invention of the EHB system allows a complete decoupling between the brake pedal and frictional brake during a small pedal-travel braking operation. Therefore, it can satisfy the blending requirements of regenerative braking and friction braking in most operating conditions, resulting in a maximum recovery of regenerative braking energy;
Advantageously, the present invention of the EHB system provides electric power assist during a large brake-pedal travel braking condition. With a feature of a force amplification by a manual boost cylinder, electric motor of a smaller power rating may be used compared with other EHB of prior art;
Advantageously, the present invention of the EHB system has a natural fail-safe function that does not require an additional mechanism for fail-safe braking operation;
Advantageously, the present invention of the EHB system has a natural brake-pedal feel function that does not require an additional mechanism to implement the brake-pedal feel simulator;
Advantageously, the present invention of the EHB system can provide a good brake pedal feel using active pressure control via precise torque control of the electric motor to eliminate an impact on the low-pressure chamber of the master cylinder without the need of a high-cost hydraulic mechanism;
Advantageously, the present invention of the EHB system allows use of traditional brake components and the traditional master cylinder with two pressure chambers in series; thus reducing the system manufacturing cost with enhanced system performance; and
Advantageously, the present invention of the EHB system allows the use of the electric motor to provide active brake control without the need of any part of pressurizing mechanism of a hydraulic control system; thus reducing the system manufacturing cost with enhanced system performance.